STUDIES OF COMPRESSION FAILURES

AND THEIR DETECTION IN

LADDER RAILS

Original report dated February 1949

Reprinted 1962

No. 1733
STUDIES OF COMPRESSION FAILURES AND THEIR

DETECTION IN LADDER RAILS1

By

MAXON Y. PILLOW, Wood Technologist

Forest Products Laboratory,2 Forest Service

U. S. Department of Agriculture

----

Some Characteristics of Compression Failures

Compression failures are minute fractures or short, locally deformed areas

along the cell walls of fibers that result when excessive endwise-compression

forces are applied to wood. Occasionally, well-defined buckling can be seen

without magnification, but, more frequently, compression failures are so small

that a high-power microscope is necessary to detect them. Readily visible

ones are shown in figure 1, as well as some that are barely visible, even at

slight magnification, when photographed under optimum conditions of lighting.

These localized areas of damaged fibers, each of which extends across at least

several annual rings, constitute one important cause of the condition in wood

commonly known as brashness, which permits pieces of lumber to break suddenly

and completely at relatively small loads and deflections and with an abrupt

fracture across the grain.

Cause of Ladder-Rail Breakage

Requests for diagnosis of causes of unexpected breakage of ladder rails during

service are frequently received by the Forest Products laboratory. These

requests come from ladder manufacturers and users, as well as from insurance

companies and others concerned with ladder failures. Examinations at the

Laboratory of many broken ladder rails showed that the fractures had occurred

at compression failures that were present before the rails ultimately broke

in what was frequently reported to have been ordinary use.

Ladder materials are specified as to kind, size, and quality in safety codes

of "American Engineering and Industrial Standards" and by the National Safety

Council for the purpose of providing equipment that is as safe as possible.

Such codes for ladder rails are based on available information regarding

defects and on characteristics of the wood that are known to affect adversely

1Report originally dated February 16, 1949.

2Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

Rept. No. 1733
the strength properties of wood in service. Previous studies of causes of

brashness in wood showed that compression failures seriously reduce bendingstrength properties, particularly when loads are applied suddenly.3 For this

reason, the safety codes require that side rails shall be free from compression

failures.

Need for Information

The objective of studies covered by this report was to provide information

essential to manufacturers and users of ladders on effects and detection of

compression failures in ladder-rail material, both during manufacture and

service of the finished products. Studies of broken parts of ladders have

demonstrated the need of understanding the nature of compression failures and

of adopting practical methods by which to exclude them from equipment on

which safety of life and property depends. Investigations of and experience

with the specifications and selection of material for aircraft during the war

resulted in the improvement of inspection methods adequate for the detection

of compression failures.4

Accordingly, the work at the Laboratory covered by this report included examinations of broken ladder rails and of material during manufacturing, determination of pertinent strength properties of the material with respect to the

presence of compression failures, further development of practical inspection

methods, and assembly of explanatory material to aid and guide inspectors in

detection of obscure compression failures. This work was facilitated by

cooperation of ladder manufacturers from whom experimental material was

obtained, and at whose plants practical inspection techniques were explored.

Examinations of Broken Ladder Rails

Ladder rails that failed in service usually had abrupt breaks across the grain

rather than splintering breaks. Microscopic examinations under polarized light

on thin sections of wood, which included the ultimate abrupt breaks, showed

that the fractures across the grain had occurred at compression failures.

These observations conformed to those of previous work on causes of brashness

that showed positive relationships between the presence of compression failures

and abrupt breaks across the grain.3

Two breaks due to compression failures occurred about 1 foot apart in one ladder

rail (fig. 2), while in another ladder both rails had broken simultaneously at the

3Cause of Brashness in Wood, by Arthur Koehler. USDA Tech. Bull. 342, 1933,

Washington, D. C.

4Detection of Compression Failures in Wood, by Eric A. Anderson. Report No.

1588, June 1944, Forest Products Laboratory, Madison 5, Wis.

Rept. No. 1733 -2-
same rung insertion because of compression failures in both rails. In the rails

of both of these ladders, the compression failures were too small to be detected by ordinary visual inspection of the sides and edges of the rails.

Their presence, nevertheless, was indicated by areas of broken fibers that

extended across several annual rings on end-grain surfaces (fig. 3) that

were cut from near the failures of each of the rails,4 Thus, the abrupt

fractures across the grain of the rails were found to occur at compression

failures already present in the wood, and the presence of these defects was

indicated in nearby parts of the rail by fiber breakage in end-grain surfaces,

although they were not detected by ordinary visual inspection of other surfaces.

Studies of Ladder-Rail Material

The numerous requests for information regarding causes of, and subsequent

determination of effects of, compression failures in relation to ladder-rail

breakage during service indicated a need by ladder manufacturers for information

on practical inspection methods by which to detect compression failures.

In order to satisfy such a requirement, additional investigation of occurrence

of compression failures in ladder-rail material and of their effect on strength

properties of the wood was considered necessary.

Material Examined

This study was made on ladder-rail material in the process of manufacture so

that observations and results would directly apply to the particular needs of

ladder producers for information on compression failures. Visual examinations

of the end-grain surfaces showed fiber breakage that indicated the presence

of compression failures in an appreciable percentage of a large number of ladder

rails in storage. Such fiber breakage had occasionally occurred on only one

end of some rails, but more frequently it was observed at both ends of the rails.

This observation indicated that compression failures were distributed throughout

the lengths of most of the rails showing such fiber breakage.

Many of the compression failures were undoubtedly small and, therefore, potentially hazardous, since they could not be readily seen and might remain undetected until after the ladders were placed in service.

Occasionally, compression failures were also visible on surfaces of ladder

rails, but the ones observed were generally very small. They were most easily

detected in rails to which a clear lacquer had been applied. Such a finish

acts as a clearing agent by reducing the refraction of light from very small

surface irregularities, such as small compression failures, so that their

characteristics are somewhat more readily identified.

Rept. No. 1733 -3-
Selection of Test Material

Some of the ladder rails examined for fiber breakage on end-grain surfaces

were selected for study by means of a toughness test to determine their

strength properties and manner of failure. Seven ladder rails that showed

fiber breakage at one or both ends, were available for this work in comparison

to six other rails on which no fiber breakage was observed. The ladder

rails had been shaped to their final cross-sectional size and were either 14

or 16 feet in length, and their surfaces had been sanded.

One sample about 2 feet along the grain was cut from an end selected at random

from each of six rails on which no fiber breakage was observed on end-grain

surfaces. From each of the seven other rails that showed some fiber breakage,

one or more samples, 2 feet long, were cut from each end. Usually, three

or four consecutive samples from one end and one sample from the opposite end

were cut from the latter rails. In addition, consecutive samples also were

cut from the complete lengths of two of these rails.

Each of the samples provided material for determination of properties of the

wood that were considered important for studying the effects of compression

failures in material for ladder rails.

Properties Studied

Previous work had shown that when compression failures were present, they

seriously affected strength, particularly that of specimens subjected to impact

loads.3 Such test specimens break suddenly and completely and with abrupt

fractures across the grain; while specimens not having compression failures

usually splinter, and the parts on each side of the break often hang together

tenaciously. Toughness-test values, as determined by a machine developed at

the Forest Products Laboratory, have been shown to be suitable for detecting

brashness in wood, particularly when this condition has been caused by compression failures that were present prior to the testing.5 Therefore toughnesstest values were determined on three specimens cut from each of the samples

obtained from the ladder rails.

Specific-gravity determinations were made on each of the toughness-test

specimens, since it is known that wood with appreciably lower density than

the average for a species is one of the causes of brashness in wood.3 The

moisture content of the material was relatively uniform as a result of conditioning the specimens under controlled conditions of temperature and relative

humidity. These values were mostly between 12 and 13 percent of the ovendry

weight.

5Strength and Related Properties of Wood Grown in the United States, by L. J.

Markwardt and T. R. C. Wilson. USDA Tech. Bull. 479, 1935. Washington, D. C,

Rept. No. 1733 -4-
End-Grain and Surface Examinations

End-grain surfaces of the samples were examined for the presence or absence

of fiber breakage. Pieces about three-fourths inch along the grain were

sawed from one end of each sample in order to standardize the examination of

the ladder-rail material. While previous observations indicated that the

relative coarseness or sharpness of a cut-off saw did not seriously hamper

such an examination, it was found to be facilitated by using a reasonably

sharp saw pulled through the piece at a moderate rate of speed. Some observations have indicated that coarse and dull saws tend to cause rougher surfaces, particularly when the saws are pulled rapidly through the piece, so

that fiber breakage will be partially obscured by the roughness of the surface. A 16-inch novelty cut-off saw, with four teeth and one raker to each

set, that was driven at about 1,800 revolutions per minute, was used. Thus,

end-grain surfaces were obtained by conditions that were considered to be as

favorable as possible for practical means of detection of fiber breakage.

Comparisons of the original ends of the ladder rails to those obtained subsequently in the Laboratory also showed that some suspected fiber breakage

was an artifact due principally to roughened surfaces produced by a coarse

and probably somewhat dull cut-off saw.

The samples were classified with respect to the presence or absence of fiber

breakage after examination under uniform conditions (table 1). This

classification, in general, conformed to the previous observation in which

fiber breakage had been suspected on one or both ends of the ladder rails

originally examined. Samples from other rails showed no indications of fiber

breakage either in the original examination at the manufacturing plant or in

that at the Laboratory under more favorable conditions (fig. 4).

Examinations at low magnification (20X) of end-grain surfaces with fiber

breakage showed that cells of the wood had been broken off cleanly rather

than having been cut and pushed to one side by the saw teeth (fig. 5). This

breakage occurred in radially oriented areas that frequently extended across

both springwood and summerwood of two or more annual rings. The saw cut

obviously must be made close to compression failures to cause such fiber

breakage. The occurrence of fiber breakage at both ends of the ladder rails

and at frequent distances along their lengths indicated that compression

failures were closely spaced and uniformly distributed throughout some of the

ladder rails being studied (fig. 6).

The fiber breakage showed considerable variation with respect to the size of

the areas in which the cells had been broken rather than having been normally

cut by the saw teeth, and also to the frequency of occurrence of broken-fiber

areas on the end-grain surface (fig. 7). The occurrence of any fiber breakage is considered evidence of the presence of compression failures. Examinations of broken ladder rails under a high-power microscope and polarized

light, both in this study and in previous ones, showed that numerous compression failures of microscopic size were present in the vicinity of fiber breakage on end-grain surfaces of the wood.

Rept. No. 1733 -5-
The surfaces and edges of the ladder-rail samples also were examined for

compression failures large enough to be detected visually by only very low

magnification. Previous studies had shown that some compression failures

could be seen when incident light struck the surface of the piece from one

side at an angle of about 20" and when the point of view was between 43° and

vertical to the surface3 (fig. 8). A 3-inch reading glass with about 2X

magnification was also used.

Of the 34 samples of ladder rails showing fiber breakage, small, but nevertheless visible, compression failures were observed in 13 samples. These

visible failures were distributed so as to include all but one of the seven

rails in which fiber breakage was found on the end-grain surfaces (table 1).

Most of these compression failures, however, were so small that they were

detected only by means of careful visual examination under optimum lighting

conditions and low-power magnification after their presence was indicated by

fiber breakage on end-grain surfaces. Thus it appears that, for purposes of

practical methods for detecting compression failures in ladder rails, careful

examinations of the ends for fiber breakage affords the greatest possibility

of identifying pieces in which this defect is present. Certainly, any rails

with visible compression failures on either edges or surfaces also should be

excluded from the finished ladders, since it is possible that they are so

localized that they did not cause fiber breakage on end-grain surfaces of the

rails. Excessive endwise compression due to rough handling, such as by dropping lumber across other material or otherwise bending it, sometimes causes

such localized compression failures. These frequently are so large, however,

that they are readily detected by ordinary visual examination of smoothly

planed and finished surfaces when suitable lighting is used.

Results of Determination

Specific gravity .--The average specific-gravity value of the specimens from

each of the samples having fiber breakage on end-grain surfaces was only very

slightly lower than that of specimens from the samples without this characteristic, namely, 0.422 and 0.431, respectively (table 1). Thus, exceptionally low specific-gravity values for the species were not found to be a cause

of brashness in either of these groups of specimens.

Toughness .--Toughness-test values were determined for three specimens from

each of the samples obtained from the ladder rails available for this study.

The average value for all of the specimens from samples having fiber breakage

was less than one-half of that of specimens from the samples without this

characteristic, namely, 53.7 and 117.4 inch-pounds per specimen, respectively

(table 1). The ranges of these values overlapped slightly; but, nevertheless,

they differed significantly (fig. 9). This overlapping of the ranges resulted

because the low specific gravity of three specimens from one sample without

fiber breakage caused moderately low toughness values and several specimens

with relatively high specific gravity caused about the same toughness values,

even though fiber breakage and compression failures were present in the

Rept. No. 1733 -6-
samples. The average toughness value of three specimens from each sample

having fiber breakage was considerably lower in relation to average specific

gravity of the specimens than that of the other specimens from samples without fiber breakage (fig. 10). Thus, for wood of the same density, fiber

breakage on end-grain surfaces is identified with seriously lower toughnesstest values than is wood without this characteristic, which is caused by the

presence of compression failures.

Types of failure. --Toughness specimens of this study, and also of previous

studies of causes of brashness,3 have shown that abrupt or brittle breaks

across the grain are very frequently caused by compression failures present

before the ultimate fractures occurred. Examination of such brittle fractures by high-power magnification has shown that many occurred at compression

failures (fig. 11). The specimens from samples having fiber breakage on endgrain surfaces had characteristically brittle failures in contrast to the

splintering failures of specimens from samples without fiber breakage (figs.

12, 13, and table 1).

Abrupt breaks due to compression failures in the specimens occurred almost

directly across many fibers and sometimes across parts of several annual

rings. Examinations by low magnification of specimens with brittle fractures

due to compression failures showed that they had characteristically steplike

contours (fig. 14). This type of failure in wood bent as a beam results when

the ultimate fracture occurs along several closely adjacent compression

failures. Such steplike breaks serve as a means of identification of

ultimate failures through previously existing compression failures. They

were found to be present, wholly or in part, in all of the specimens from

samples showing fiber breakage on the end grain, but not in those from

samples that did not show this characteristic (table 1).

Thus these data, in combination, showed that brittle failures and low toughness values were caused by compression failures and were related to the

occurrence of fiber breakage on end-grain and, occasionally, to visible compression failures on side surfaces. Specimens with splintering failures and

relatively higher toughness values were obtained only from samples of ladder

rails on which no evidence of compression failures was observed by visual

examinations of ends and sides of the samples.

Distribution of Compression Failures

This study showed that compression failures were abundantly distributed along

the length of the 2-foot samples having fiber breakage on their end-grain surfaces. The fiber breakage was at least 6 inches and was sometimes more than

1 foot from the brittle failures of specimens also having low toughness

values caused by compression failures. Thus the several samples that were

obtained from a single ladder rail had fiber breakage, brittle fractures, and

low toughness values that demonstrated an abundant occurrence of compression

Rept. No. 1733 -7-
failures distributed at very short intervals along the length of the rail,

Many of these were of microscopic size, since the only visible compression

failures that were found in these rails were widely scattered and relatively

few in number. In fact, they were detected only after careful examination

under highly favorable conditions.

Occasional pieces of lumber have been found to have visible compression

failures that were restricted to a narrow zone across the piece. Such pieces

did not show fiber breakage except close to the obviously damaged zone. It

is believed that such compression failures are due to rough handling of the

lumber, such as by dropping it across obstacles, that causes buckling of the

wood in localized areas. Many compression failures, however, are believed to

result from more widely distributed and possibly less severe stress, so that

microscopic buckling of the fibers occurs abundantly along the length of a

piece of lumber. Stresses in standing trees resulting from violent winds are

believed to contribute to wide distributions of compression failures such as

were observed in this study.

Recommendations for Inspection Practices

The study of ladder rails and previous ones on causes of brashness of wood

have shown that compression failures are a serious hazard to strength properties of materials on which safety of life and property depends. These

studies also have shown that the presence of compression failures can be detected by practical inspection methods by which unsuitable pieces can be excluded from critical equipment. Observations made under manufacturing conditions and on samples submitted to the Forest Products Laboratory have demonstrated that compression failures of microscopic size are exceedingly numerous

in some pieces of material, while those large enough to be visible without

high-power magnification are rare. It has usually been the effects of compression failures on strength properties and on types of failures that have

been observed.

Inspection of End-Grain

Observations at plants manufacturing ladder rails showed that careful inspection of end-grain surfaces for fiber breakage is a practical means of detecting compression failures. Such inspections can be made on sawed lumber in

large sizes intended for resawing to ladder-rail dimensions, provided that

the ends are sawed with reasonable smoothness. Exclusion of pieces showing

fiber breakage in rough-lumber sizes will result in some saving in manufacturing costs (fig. 15, A). Lumber having that characteristic confined to only

part of the cross section sometimes can be resawed so as to salvage the remainder that is suitable for ladder-rail material (fig. 15, B).

Rept. No. 1733 -8-
ends of ladder rails, when trimmed to final length, should be cut with a saw

that will produce a surface having smoothness suitable for detecting fiber

breakage due to compression failures. A coarse, dull saw has been found to

obscure fiber breakage, while a very fine, sharp saw is inclined to produce

somewhat glossy surfaces having a similar effect on identifying fiber breakage. Inspection of ladder-rail ends should be made directly after the ends

are trimmed, or when they are piled in storage if ladder assembly is not to be

completed immediately. Ladder rails piled on a push truck with the ends exposed to moderately bright light coming from one side is a condition recommended for a rigid inspection by which to identify fiber breakage caused by

the presence of compression failures. Diffused light, regardless of its

brightness, and coming from all sides so as to dissipate shadows, is not

satisfactory.

The inspection for fiber breakage should be rigid, and both ends of each rail

piled on a truck or in a storage pile should be inspected. Any rail having

fiber breakage on both ends should be excluded from assembly into a ladder.

Rails showing fiber breakage on only one end can be shortened at about 2-foot

intervals. If the fiber breakage on one end of such a rail completely disappears within a relatively short length, for example, 2 to 6 feet, then it

is believed that the remaining length can be used with reasonable safety,

provided there are no other visible compression failures on the sides or

edges.

The known information on occurrence and effects of compression failures

demonstrates that a ladder rail having fiber breakage on one or both ends is

likely to be hazardous to safety of life and property. Therefore, ladder

rails having fiber breakage at either end, such as is shown in figures 3, 6,

and 7, are not considered safe.

Inspection of Sides

Sides and edges of all ladder rails should also be inspected for identification of any visible compression failures that may be present. These may be

obscured by unevenness of sawed or even of planed surfaces. Compression

failures are made more readily visible, however, after clear lacquer, shellac,

or varnish has been added to the surfaces. Application of a nonswelling

liquid, such as carbon tetrachloride, also makes small, but not microscopic,

compression failures more readily visible.

Careful inspection of sides and edges of ladder rails after they have been

lacquered or varnished, is considered a practical means of detecting visible

compression failures that may have resulted from localized endwise compression

stresses that resulted in forming compression failures only in narrow zones

across the width of the rails. Incident light from a reflector spot bulb, for

example, focused at an angle of about 20° to the surface being examined, has

been found to facilitate identification of visible compression failures.

These are most readily seen when viewed from a position between 45° and 90°

with respect to the surface and from the same side from the vertical as the

source of light (fig. 8).

Rept. No. 1733 -9-
Arrangement of Lights

Figure 16 shows a convenient arrangement of lighting from a reflector

spotlight for identifying visible compression failures on the side grain and

from a conventional incandescent 60-watt bulb in a desk lamp for examining

end-grain surfaces for fiber breakage. The latter bulb should be shielded

so that the light does not shine directly in the inspector's eyes. Studies

of these methods to identify visible compression failures and fiber breakage

have shown that inspections can be carefully and rapidly made on ladder rails

on a table of convenient height for the work. A cleat about 3/4 inch thick

and 1-1/2 inches wide nailed to the table top was found to facilitate turning

the pieces during examination of all four sides.

In general, it is recommended that the actual inspections of end-grain and

side surfaces be made by inspectors who are qualified by understanding and

practical knowledge of the appearance, characteristics, and effects of compression failures on the strength properties of wood. Thus responsibility

for inspection can be assigned to qualified supervisory or experienced

personnel having other duties, and thereby interference with production can

be minimized. Workers making the final assembly of the ladders, however,

always should be alert to detect any visible compression failures that may

appear as lines of buckled fibers across the grain of ladder rails. Compression failures have been observed in ladders being assembled,' although it was

not known whether they might have been detected earlier by adequate inspection or were only made visible by the stresses applied during assembly.

Rept. No. 1733 -10- .4-28.
Table 1.--Occurrence of fiber breakage on end-grain and readily visible compression failures on side surfaces

of ladder rail samples, average specific gravity and toughness test values, and descriptions of the

types of test failures of three specimens from each sample.
Figure 1 .--Compression failures in spruce lumber that are visible

as lines across the grain. Slightly magnified.

Z M 80312 F
Figure 2.--Side view of a ladder rail that failed in two places

at approximately the same time because compression failures were

present prior to the ultimate breaks in tension, which are indicated by arrows.

ZM 80313 F
Figure 3. --Cross section of a ladder rail showing fiber breakage

that indicates the presence of compression failures.

Z M 80314 F
4.--Cross sections of ladder-rail samples that showed no

fiber breakage.

Z M 80315 F

Figure
Figure 5. --Photomicrograph of fiber breakage due to compression

failures, showing areas of cleanly broken fibers interspersed by

groups of other fibers cut by the saw teeth and pulled to one

side. x20.

Z M 8O316 F
Figure 6.--Four cross sections cut at about 2-foot intervals from

one end (13) and one from the opposite end (14) of a 16-foot

ladder rail, all showing fiber breakage caused by the presence

of compression failures.

ZM 80317 F
Figure 7 .--Cross sections from a ladder rail cut at about 2-foot

intervals, all of which show fiber breakage to lesser or greater

extents, indicating the presence of uniformly distributed compression failures along the length.

Z M 80318 F
ZM 54161 F

Figure 8 .--Most effective angle of view and angle between wood surface and incident

light when searching for compression failures.
Figure 9.--Total range, range of the middle two-thirds of the specimens (crosshatched), and average of toughness-test values of specimens obtained from samples

of ladder rails, some that showed fiber breakage on end-grain surfaces and, also,

other samples without this characteristic.
Figure 10.--Relation of average toughness-test values to specific

gravity of three specimens from each sample with fiber breakage

In comparison to similar values of specimens from samples without

fiber breakage.

ZM 80320 F
piece ultimately broke. X450 by polarized light.

occurred at a compression failure already present before

Figure 11.--Photomicrograph of a brittle break in tension

M 80321 F

that

the
Figure 12.--Brittle breaks across the grain of toughness-test specimens from a ladder-rail sample that showed fiber breakage on

end-grain surfaces.

Z M 80322 F
Figure 13 .--Splintering toughness-test failures of specimens from

sample without fiber breakage on end-grain surface.

Z M 80323 F
Figure 14.--Steplike break in a toughness-test specimen that is

characteristic of a fracture through several compression failures

already present before the ultimate break occurred. X5 photographed by reflected light.

Z M 80324 F
Figure 15.--End-grain surfaces of Sitka spruce lumber showing fiber

breakage: A, distributed across most of the surface; and B, in

the part below the black line in comparison to wood free from this

characteristic above the line.

Z M 54160 F
Figure 16.--A convenient arrangement of a reflector spotlight and an ordinary

incandescent-bulb desk lamp placed on a work table for examination of ladder

rails to detect visible compression failures and fiber breakage on the sideand end-grain surfaces.

ZM 80325 F
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